DE102017109614B4 - Process for solution annealing a casting - Google Patents

Abstract

Method for solution annealing a casting consisting of an aluminum-silicon-based casting alloy, the method comprising solution treatment of the casting above an incipient melting temperature in the following steps: heating the casting to a first temperature of 495 ° C for a first period of 3 hours; heating the casting to a second temperature of 515°C for a second period of 3 hours; andheating the casting to a third temperature of 530°C for a third period of 2 hours; andwherein the cast alloy includes a composition with the following proportions by weight:from 5.00% to 17.00% silicon (Si);from 0.00% to 0.90% iron (Fe);from 0.00% to 1.00% manganese (Mn); from 0.000% to 0.018% strontium (Sr); from 0.00% to 2.00% copper (Cu); from 0.00% to 0.50% magnesium (Mg); from 0.00 % to 0.05% zinc (Zn); from 0.01% to 0.10 boron (B); anda remainder of aluminum (Al).

Description

TECHNICAL FIELD

The present disclosure relates to a method for solution annealing a casting consisting of an aluminum-silicon-based casting alloy.

BACKGROUND

The statements in this section merely provide background information pertaining to the present disclosure and may or may not reflect the prior art.

Al-Si-based cast aluminum alloys find widespread applications for components in the automotive, aerospace and aerospace industries due to their good castability, corrosion resistance, machinability and high strength-to-weight ratio. Regarding castability, alloy compositions with lower silicon content were believed to produce inherently poor castings due to a wider freezing range and reduced latent heat. Alternatively, alloy compositions with higher silicon content become increasingly difficult to machine and exhibit lower ductility and fracture toughness due to coarser primary silicon particles. In general, aluminum alloy casting performance is based on several factors including alloy composition, casting and solidification conditions, and post-casting process or heat treatment.

In attempting to expand or improve the uses of aluminum alloys in additional applications to exploit the benefits of aluminum alloys, previous aluminum alloy casting compositions and processes have proven unsuccessful in high temperature applications. The overwhelming problem with using aluminum alloy castings in high temperature applications is the tendency for the material to change properties during operation. When designing castings for these applications, one of the most important aspects of material properties is precisely that the material properties remain unchanged during operation. However, for this purpose, currently available commercial aluminum alloys do not offer such material property stability.

DE 36 32 609 A1 concerns high-strength aluminum alloys for press casting. US 2009 / 0 010 799 A1 relates to a cast aluminum alloy. WO 97/13 882 A1 relates to a method for reducing the formation of primary plate-shaped beta phase in iron-containing AlSi alloys. DE 10 2013 212 439 A1 concerns cast aluminum alloys for structural components.

Accordingly, there is a need in the art for a manufacturing process that exhibits improved initial material properties while maintaining or stabilizing the material properties throughout the life of the casting in a high temperature application.

SUMMARY

The present invention provides a method for solution annealing a casting consisting of an aluminum-silicon-based casting alloy according to claim 1. The cast alloy has a composition that has a weight proportion of 5.00% to 17.00% silicon (Si), 0.00% to 0.90% iron (Fe), 0.00% to 1.00% manganese (Mn); from 0.000% to 0.018% strontium (Sr), from 0.00% to 2.00% copper (Cu), from 0.00% to 0.50% magnesium (Mg), from 0.00% to 0.05 % zinc (Zn), from 0.01% to 0.10% boron (B); and has a residue of aluminum (Al). In the present invention, the casting is solution treated over an initial (beginning) melting temperature by heating the casting to a first temperature for a first period of time, heating the casting to a second temperature for a second period of time, and heating the casting to a third period of time is heated to a third temperature. In the present invention, the first temperature is 495°C and the first period is three hours, the second temperature is 515°C and the second period is three hours, and the third temperature is 530°C and the third period is two hours.

In another example of the present invention, the composition includes a weight level of from 7.85% to 7.90% silicon and from 0.20% to 0.30% iron.

In another example of the present invention, the composition includes a weight level of 0.00% strontium.

In another example of the present invention, the composition includes a weight level of 0.009% strontium.

In another example of the present invention, the composition includes a weight level of 0.40% to 0.41% iron and 0.00% strontium.

In another example of the present invention, the composition includes a weight level of greater than 0.25% magnesium.

In another example of the present invention, the composition includes a weight level of greater than 1.50% copper.

In another example of the present invention, producing a casting includes a sand casting process, a permanent mold casting process, a semi-permanent mold casting process, a high pressure casting process, a clamp casting process, and foam casting processes.

In another example of the present invention, the casting is analyzed to determine a silicon particle volume fraction, an average silicon particle size, an insoluble intermetallic particle volume fraction, and an insoluble intermetallic average particle size.

In a further example of the present invention, the casting is aged at a temperature between 150 ° C and 190 ° C and within a period of 6 to 10 hours.

The foregoing functions and advantages, as well as other functions and advantages of the present invention, will be apparent from the following detailed description of the best possible practice of the illustrated invention, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

• 1 zeigt ein Diagramm, das die Raumtemperatur-Zugfestigkeit für unmodifiziertes 319 Al bei unterschiedlichen Wärmebehandlungen gemäß einem Beispiel der vorliegenden Erfindung darstellt;
• 2 zeigt ein Diagramm, das eine Zugfestigkeit von erhöhter Temperatur (250 °C) für unmodifiziertes 319 Al bei verschiedenen Wärmebehandlungen gemäß einem Beispiel der vorliegenden Erfindung darstellt;
• 3 zeigt eine Reihe von Mikrofotografien verschiedener Aluminium-Silizium-Gussteile gemäß der vorliegenden Erfindung;
• 4 zeigt eine Reihe von Mikrofotografien verschiedener Aluminium-Silizium-Gussteile gemäß der vorliegenden Erfindung;
• 5 zeigt eine Reihe von Mikrofotografien verschiedener Aluminium-Silizium-Gussteile gemäß der vorliegenden Erfindung;
• 6 zeigt eine Reihe von Mikrofotografien verschiedener Aluminium-Silizium-Gussteile gemäß der vorliegenden Erfindung;
• 7 zeigt ein Diagramm, das die Raumtemperatur-Zugfestigkeit des unmodifizierten 319 Al, Fe-modifizierten 319 Al und Sr-modifizierten 319 Al gemäß einem Beispiel der vorliegenden Erfindung durch eine Wärmebehandlung verarbeitet darstellt, und
• 8 zeigt ein Diagramm, das die Raumtemperatur-Zugfestigkeit des unmodifizierten 319 Al, Fe-modifizierten 319 Al und Sr-modifizierten 319 Al aus Tabelle 1, welche gemäß einem Beispiel der vorliegenden Erfindung durch eine Wärmebehandlung verarbeitet wurden, darstellt.

The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present disclosure in any way.

• 1 Fig. 12 is a graph illustrating room temperature tensile strength for unmodified 319 Al under different heat treatments according to an example of the present invention;
• 2 Fig. 12 is a graph illustrating an elevated temperature (250°C) tensile strength for unmodified 319 Al at various heat treatments according to an example of the present invention;
• 3 shows a series of photomicrographs of various aluminum-silicon castings in accordance with the present invention;
• 4 shows a series of photomicrographs of various aluminum-silicon castings in accordance with the present invention;
• 5 shows a series of photomicrographs of various aluminum-silicon castings in accordance with the present invention;
• 6 shows a series of photomicrographs of various aluminum-silicon castings in accordance with the present invention;
• 7 Fig. 12 is a graph showing the room temperature tensile strength of the unmodified 319 Al, Fe-modified 319 Al and Sr-modified 319 Al processed by heat treatment according to an example of the present invention, and
• 8th Fig. 12 is a graph showing the room temperature tensile strength of the unmodified 319 Al, Fe-modified 319 Al and Sr-modified 319 Al from Table 1, which were processed by heat treatment according to an example of the present invention.

DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or use in any way.

As with most metal casting practices and applications, the microstructure of the metal alloy has a large influence on the mechanical properties of the material. Specifically, when treating Al-Si alloys, the primary casting practices that affect microstructure are solidification rate, chemical impurity modification, and thermal modification during heat treatment. For almost all compositions, the minimum increased temperature strength achieved, particularly after long-term elevated temperature exposure, is higher for parts cast in Al-Si alloys that have not been subjected to solution heat treatment. Additionally, higher volume fractions of eutectic Si are another way to maintain increased temperature strength. However, both the absence of solution heat treatment and highly eutectic Si contribute to lower heat treatment and room temperature strength.

With reference to the 1 and 2 The tensile strength is plotted for several samples of the Al-Si alloys with varying Si and Sr content. 1 shows the room temperature tensile strength while 2 shows an increased temperature (of 250 °C). Overall, the increased temperature tensile strength decreases significantly with each heat treatment. Even more revealing is that the solution-cast castings (T6 and T61-2) lost significantly more of their increased temperature strength than the merely precipitation-treated or as-cast castings (AC, T52, T56).

With reference to 3 Unmodified 319 Al photomicrographs show the changes in the morphology of the eutectic silicon particles due to increased temperature loading by solution heat treatment at 495 °C. The cast sample a) reflects the plate-like eutectic silicon. After one hour of solution b), the eutectic silicon particle size decreases with a certain rounding of the ends of the particles. As dissolution time c) and d) increases, the particle size further decreases, while the eutectic silicon particle shape becomes significantly more rounded as spheroidization continues.

Next includes 4 Photomicrographs for a strontium-modified 319 Al microstructure after solution heat treatment for a) as-cast, b) 1 hour, c) 16 hours, and d) 48 hours. Strontium modification and high cooling rates lead to finely divided eutectic silicon in the as-cast state a). At the start of the solution treatment, the particle size decreases after one hour b). However, as the solution time increases to 16 c) and 48 hours d), the particle size becomes coarser and more rounded.

As can be concluded from the above results, solution treatment of unmodified and strontium-modified 319 Al tends to produce a similar microstructure of coarse, rounded eutectic silicon. However, 48 hours is too long for commercial casting production. A route to the microstructure that is achieved within 48 hours of solution treatment must be created.

Continuing the search for stable aluminum castings in a high-temperature operating environment, the effects of composition modification and solution temperature on the microstructure are investigated. Table 1 below provides the compositions of 319 Al with manganese (Mn), 319 Al with Mn modified with strontium (Sr) and 319 Al with Mn modified with higher iron (Fe) content. Table 1: Sample composition for 319 Al with modifications. 319 Al + Mn, 319 Al + Mn + Sr and 319 Al + Mn + Fe. Si Fe Cu Mn Mg Zn b Ti Sr SG 319+Mn 7,890 0.228 3,910 0.426 0.304 0.095 0.001 0.130 0.000 2,720 319+Mn+Sr 7,730 0.214 4,050 0.421 0.292 0.065 0.000 0.144 0.009 2,710 319+Mn+Fe 7,860 0.405 3,770 0.459 0.283 0.100 0.001 0.124 0.000 2,700 Table 2: Heat treatment steps and temperatures of the sample solution. Heat treatment 495°C 515°C 530°C 540°C 555°C Old In cast condition no no no no no no T6 8:00 a.m 180 °C 8:00 a.m T61 3:00 a.m 5:00 a.m 180 °C 8:00 a.m T62 3:00 a.m 3:00 a.m 2:00 a.m 180 °C 8:00 a.m T63 3:00 a.m 2:00 a.m 2:00 a.m 1:00 a.m 180 °C 8:00 a.m T64 3:00 a.m 2:00 a.m 1:00 a.m 1:00 a.m 1:00 a.m 180 °C 8:00 a.m

With reference to the 5 and 6 Shown are the resulting photomicrographs from samples modified with the compositions of Table 1 and solution annealed at the times and temperatures indicated in Table 2. As can be seen, when the maximum solution temperature exceeds 515 °C, the rounding of the eutectic silicon particles is accelerated. It is estimated that for every 10 °C increase in temperature, the time required for the particles to round is halved. How, however 6 As can be seen, initial melting occurred during the heat treatment of the T64 solution, increasing the solution temperature to 555 °C and triggering an adverse reaction. Nevertheless, a development should be noted here in which the average size of the silicon particles initially decreases minimally before the start of a coarsening process. This coarsening process continues until the 540 °C minimum before initial melting overtakes the coarsening process. In addition, regarding Fe modification, the Fe containing insoluble intermetallic particles is rounded and fragmented into smaller particles as the solution temperature is increased. This opens the possibility for initial melt-free microstructures at higher temperatures; at least up to the eutectic temperature.

Tensile strengths from the samples 5 and 6 are in the 7 and 8th displayed graphically. It is important to note that the room temperature peak strength results from the heat treatment of the T61 solution.

The tensile strength of the samples from Tables 1 and 2, which were subsequently stabilized for 200 hours at 250 °C, is in 8th shown. However, the as-cast microstructure has the best strength retention, but the high-temperature strength also begins to improve as the maximum solution temperature is increased, resulting in coarsening of the second phase particles.

A majority of the hard particles in the microstructure, the aggregate hardness and strength increase; especially at elevated temperatures, where the hardness and strength of the aluminum matrix decreases rapidly. Soluble phases such as soluble intermetallic elements and reinforcing precipitates only slightly increase the hardness and strength after long-term elevated temperature stress. However, soluble intermetallic elements and reinforcing precipitates are very helpful in manufacturing. For example, machining forces and tool wear are generally lower for aluminum castings with higher hardness. Thus, maintaining a minimum level of hardening is necessary to aid in the production of the machined casting. For long-term durability in an elevated temperature or cycling temperature environment, an optimal volume fraction of hard particles is required that balances hardness and strength against ductility and fatigue resistance. The optimal volume fraction is not fixed for every casting design application, as loading, part geometry, operating temperature, and heating rates all contribute to determining the optimal volume fraction. In addition, the nature of the hard particles, including hardness, size, shape, spacing, and interfacial bonding to the matrix, also contributes the information required to determine the optimal volume fraction of the hard particles.

As a result, a minimum level of precipitation hardening is required. In aluminum-silicon casting alloys, sufficient hardness can be developed with magnesium above 0.25% or copper above 1.5%, or a combination of both. Keeping the Mg and Cu as low as practical is important because higher levels result in the formation of soluble intermetallic elements, which reduce casting quality and compromise the heat treatment process described above by blocking initial melting.

In addition, the volume fraction, size and distribution of the insoluble hard phases of both eutectic silicon and intermetallic elements must be measured to a close tolerance level. This is achieved through chemical modification to match the local solidification conditions during the primary casting process. The volume fraction of these phases is not changed by subsequent thermal processing; just the shape and size, which is why it is beneficial to create the right amount in the initial microstructure. For high temperature applications, the alloy contains silicon in an amount between 5.0% and 17.0% by weight, iron in an amount between 0.0% and 0.9% by weight, manganese in an amount between 0.0% and 1.0% by weight, chromium in an amount between 0.0% and 0.3% by weight and nickel in an amount between 0.0% and 2.0% by weight.

Subsequent optimization of the heat treatment process produces the size, shape and volume fraction of insoluble hard particles specified for the application. Commercial alloys can exhibit improved temperature strength at elevated temperatures if they have not been solution treated because a significant proportion of the hard phases are soluble and thus can disappear during solution. Additionally, current solution treatment processes have been optimized to minimize eutectic silicon size and maximize ductility. However, slightly larger particle sizes are required for increased temperature resistance. Thus, a sub-solution that treats an alloy with fairly good morphological control of the insoluble intermetallic elements at an optimal volume fraction is one of the possible processes that can be used. Alternatively, supercritical solution treatment by scaling the temperature to levels that allow the eutectic silicon to grow beyond the minimum while simultaneously refining and spheroidizing the insoluble intermetallic phases may represent another possible method. Once the shape and size of the hard particles are defined by the solution treatment process, subsequent thermal loading during aging and service will have little effect on the properties. Therefore, maintaining a stable structure and the properties that determine the minimum hardness and strength are the goal of the process once the curing precipitates have become incoherent.

wobei sich A als gemessene Fläche des Teilchens) mit Teilchenform der Kugelform annähert. Die eutektische Siliziumphase ist inbegriffen und liegt für Anwendungen mit hoher Zähigkeit bei etwa 6 bis 12 Volumenprozent und für Anwendungen mit höherer Steifigkeit bei 6 bis 15 Volumenprozent.Alloy composition, casting process and heat treatment process are controlled to form an aluminum casting with insoluble intermetallic phases that are refined and spheroidized. Although the intermetallic phases are primarily iron based, they may contain Mn, Cr, Ni, etc. as minor impurities. The intermetallic phases are included at less than 2 to 3 volume percent for applications requiring room temperature toughness or 6 to 10 volume percent for high stiffness applications. The eutectic silicon phase is stabilized beyond the minimum particle size and spheroidized through fragmentation of unmodified structures or agglomeration of modified structures. The size of the hard particles is ideally between 50 and 110 micrometers (equivalent circle diameter $\left(\mathrm{\mu }\text{m}\right)=\sqrt{4\text{A}/\mathrm{\pi }},$

where A is the measured area of the particle) with particle shape approximating the spherical shape. The eutectic silicon phase is included and is approximately 6 to 12 percent by volume for high toughness applications and 6 to 15 percent by volume for higher stiffness applications.

Thus, Table 3 shows alloy compositions for various applications; including medium or high temperature and high or low toughness.

In addition to the composition guidelines of Table 3, Table 4 includes guidelines for optimal microstructural characteristics for each of the applications. Table 4: Ideal microstructure properties for medium or high temperature, low or high toughness applications. Optimal microstructure properties Silicon particle size □m 30-60 30-70 50-80 50-110 Silicon volume fraction % 5-6 5-8 6-12 6-15 Insoluble intermetallic particle size □m 30-55 30-80 50-65 55-75 Insoluble intermetallic volume fraction % 2-3 2.5-4.5 4-7 6-10

While the best modes of carrying out the invention have been described in detail, those skilled in the art familiar with the art described herein will recognize various alternative designs and embodiments by which the invention may be practiced within the scope of the claims set forth below.

Claims (8)

Method for solution annealing a casting consisting of an aluminum-silicon-based casting alloy, the method comprising solution treatment of the casting above an initial melting temperature in the following steps: heating the casting to a first temperature of 495°C for a first period of 3 hours; heating the casting to a second temperature of 515°C for a second period of 3 hours; and heating the casting to a third temperature of 530°C for a third period of 2 hours; and wherein the casting alloy contains a composition with the following proportions by weight: from 5.00% to 17.00% silicon (Si); from 0.00% to 0.90% iron (Fe); from 0.00% to 1.00% manganese (Mn); from 0.000% to 0.018% strontium (Sr); from 0.00% to 2.00% copper (Cu); from 0.00% to 0.50% magnesium (Mg); from 0.00% to 0.05% zinc (Zn); from 0.01% to 0.10 boron (B); and a remainder of aluminum (Al). The procedure according to Claim 1 , wherein the composition contains a weight proportion of 7.85% to 7.90% silicon and 0.20% to 0.30% iron. The procedure according to Claim 2 , wherein the composition contains a weight proportion of 0.00% strontium. The procedure according to Claim 2 , wherein the composition contains a weight proportion of 0.009% strontium. The procedure according to Claim 1 , wherein the composition contains a weight proportion of 0.30% to 0.41% iron and 0.00% strontium. The procedure according to Claim 1 , wherein the composition contains a proportion by weight of more than 0.25% magnesium. The procedure according to Claim 1 , wherein the composition contains a proportion by weight of more than 1.50% copper. The procedure according to Claim 1 , wherein producing the casting includes a sand casting process, a permanent mold casting process, a semi-permanent mold casting process, a die casting process, a clamp casting process or a foam casting process, and wherein the casting is analyzed to determine a silicon particle volume fraction, an average silicon particle size, an insoluble intermetallic particle volume fraction, and a to determine insoluble intermetallic average particle size.

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